For subsea equipment it may be a special task to be arranged such that they can operate reliably under a given pressure of sea water. Subsea enclosures for electrical or mechanical equipment can be divided in two groups, pressure resistant enclosures and pressure compensated enclosures. The latter are characterised that the enclosure is fluid filled and the pressure inside the enclosure is about the same as the environment or a little higher.
With pressure compensation it is the task to maintain sufficient pressure within the enclosure, e.g. to withstand the pressure from ambient sea water. The pressure within the enclosure may be less than the pressure of the sea water, thus a subsea equipment housing or enclosure need only withstand a pressure differential between the external pressure exerted on the enclosure by the sea water and an internal pressure which is maintained within the enclosure.
To provide the appropriate fluid pressure, in certain applications hydraulic fluid within an enclosure is pressurised by a spring.
Besides, an encapsulation of subsea equipment in a closed housing may also prevent the ingress of sea water into the system.